Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: My first observations with natural horizon
    From: Joel Jacobs
    Date: 2005 Aug 12, 05:51 +0000
    Alex,
     
    Thank you for this extensive report. It provides a valuable reference for anyone who intends to do CelNav from a smaller size platform at sea.
     
    I made particular note of your comment about the enclosed tangent screw of a trommel type sextant such as in the SNO-T/SNO-T and Freiberger. I have alays found that in time, the index arm will freeze because of an accumulatuion of salt inside the barrel unless fresh water is used to flush the mechanism. Oil will not do it.
     
    Also the limitations of the inverting scope in rough going.
     
    Also noteworthy,  the difficulties you experienced in dealing with the centrifical forces at play and in finding a secure place from which to shoot. I always found it convenient to wedge myself between two shrouds with my arm wrapped around one, or wrapped around a stay. Another spot would be standing with my back braced against the mast with my legs exerting back pressure. I always took the sight standing and from the crest of a wave never the trough. When necessary, a change of course was made to reduce rolling, pitching never seemed to bother me.
     
    Thanks agin for your interesting read.
     
    Oh yes, a lanyard secured the sextant to my neck, and most often a timekeeper was available. And frequently, I had an assistant navigator on board so that we would compare sights. That made the process entertaining as well.
     
    Joel Jacobs
    --
    Visit our website
    http://www.landandseacollection.com


    -------------- Original message from Alexandre E Eremenko <eremenko{at}MATH.PURDUE.EDU>: --------------


    > Joel,
    > Your questions give me an opportunity to describe my
    > limited experience in detail.
    >
    > 1. First I found, to my surprise, that it is hard to
    > find a place for a sextant in a 42 feet boat:-)
    > I mean, to have the sextant box in a secure place, and at the
    > same time, easily accessible. Finally I put it on a little shelf
    > (with a board) in my cabin, and secured the box with towels
    > to prevent it from moving horizontally. But opening the box, taking the
    > sextant out and storing it back was a substantial hussle.
    > I appreciated the relatively small size of the box of my sextant,
    > and I disagree with Bauer who says in his book that he prefers boxes
    > which allow the st! orage of the sextant with a telescope attached.
    > This may be a luxury for big ships, but not for a small boat.
    > (The largest boxes I've seen so far are those of C. Plath,
    > almost the size of a siutcase:-) and the smallest ones are
    > of some Hughes "Mate" sextants.
    >
    > 2. Second, a major difficulty was to carry my sextant around
    > in a rolling boat from my cabin to the cockpit and back.
    > As the boat was rolling violently, I had to use my one hand
    > to hold a rail (or someting else) balancing the sextant in another hand,
    > anxious not to kick it against anything in the sudden moves of the
    > boat. Immediately after observation, I had to carry the sextant
    > back to my cabin, because there was no secure place to put it in the
    > cockpit.
    >
    > I experimented with fixing the whole box outside, on the cabin's
    > roof, under the tent, but this did not work because it inter! fered with
    > the ropes, tent, etc.
    >
    > 3. It was very hard in the first two days indeed.
    > So hard that I had to make most observations without a scope
    > in these first days. And had hudge blunders and errors.
    > (I already mentioned that the zero-magnification tube
    > seems to be absolutely necessary for observations without
    > a scope, and I did not have it). We sailed before
    > the wind, at 6 to 8 knots, with pretty large waves,
    > possibly 10 feet by my estimate, and the
    > boat was rolling, pitching and yawing very much.
    >
    > Yawing motion is the most unpleasant for sextant observations,
    > especially with an inverting scope:-)
    >
    > 4. However I adapted quickly. (Or maybe the sea was not so rough
    > in the subsequent days). The best results were achieved when we
    > sailed to windward and rolling was not so strong.
    >
    > Anyway, in the first week! of August I was able to obtain
    > decent results with my favorite inverting scope.
    > I found that the wires help to keep the sextant vertically:
    > you align the horizontal wire with the horizon.
    > "Rocking" is more precise, of course, but it was not always possible
    > when the boat was rolling like crazy.
    >
    > 5. I found that the best position in the rough sea for observation was
    > sitting on the cabin roof close to the mast, leaning with my
    > back to the mast
    > (tied to a railing with a harness, but this was mostly for
    > phychological
    > reasons). Soon I will post a picture of myself doing this.
    >
    > The hardest part was to keep the Sun and the horizon in
    > the field of view for the time necessary for observation.
    > I understand that inverting scope makes it much harder,
    > so only now I fully understand the complains of the list members
    > about the inver! ting scope:-) The problem was not present on land.
    >
    > I believe that standing at the same place would be better,
    > but I found no way to fix my body in standing position,
    > with both hands free, and feeling stable enough.
    >
    > The reason why standing positiuon is preferred is that
    > in low position the waves frequently obscure the horizon.
    >
    > 6. On smaller waves I found it most convenient sitting on
    > the windward cockpit bench, with my feet pressing against the table
    > in the center of the cockpit (on smaller yachts, the opposite
    > bench can play this role).
    >
    > 7. The procedure was usually this. I came to the cockpit with the scope
    > detached (in my pocket). Another pocket contained a little notebook
    > and a pencil. Sextant held in the right hand,
    > and attached with a strap to my neck. For the timing, I used my
    > waterproof wristwatch.
    > > First I put the darkest shade on the horizon mirror
    > and aim it at the sun (without the scope) holding the sextant
    > upside down. Keeping the Sun in view, I moved the arm until I could
    > see the horizon. Then I attached the scope, engaged a dark filter
    > on the index mirror and changed the filter on the horizon mirror
    > for a ligter one. Keeping the sextant upright, now I tried to achieve
    > a decent touch. (But of course both the Sun and the horizon try
    > to escape all the time from my field of view in all directions:-)
    > After I had a satisfactory touch, I looked at the watch memorizing the
    > seconds first. Then, sextant hanging on a strap on my neck,
    > I pulled the notebook and pencil, recorded the time, and after that,
    > the sextant reading. This I tried to repeat 3-5 times, until
    > the Sun hided behind the clouds, or my hand got tired
    > (I really appreciate the light weight! of my sextant).
    > Then I had to walk and record the GPS reading for control.
    >
    > 8. Very frequently the sextant was splashed by sea water in the process.
    > So after each observation I had to rinse the mirrors and the shades
    > with fresh water, and add a little oil to the drum.
    > (Otherwise, next time the drum assembly will be stuck and will
    > move only with an effort. First time it was stuck so strongly that I had
    > to add oil before I could move it.)
    >
    > But even during the observation, the water drops on the mirrors
    > and shades sometimes had to be wiped because they interfere with the view.
    >
    > 9. To answer your question, I never asked to change the course,
    > or anything else, to accomodate my observation. And never used
    > an assistant, even to look at GPS.
    > It was important who steered the boat: with a more experienced helmsman
    > the boat yaws much les! s, and yawing was a major nuisance, as I said
    > before.
    >
    > Following the Russian manual I observed the stars with my upright scope
    > and Sun with the inverting scope. But only once there was an opportunity
    > to observe the stars.
    >
    > Besides my sextant and watch, I carried only a notebook,
    > a pencil, the Almanac and Casio calculator.
    > (No reduction tables).
    >
    > 10. I consider the enclosed screw a drawback of SNO/Freiberger
    > sextants. Maybe it protects from dust but not from the splashes of
    > sea water. And you cannot wipe the mechanism, and you cannot even see it.
    > The only thing you can do is to add some oil.
    >
    > 11. My "dream sextant" will have three scopes: a straight Galileo one
    > some 3.5 times 50, an inverting one with wires, 6 to 8 times 40,
    > a zero magnification tube (or better a "pinhole" of the old British
    > sextants), several e! yepieces for various magnification,
    > and an eyepiece filter. It will have an open screw assembly,
    > classical split view mirror, and a small box.
    >
    > I am aalso very curious to try a sextant with a nonius assembly
    > instead of the worm screw. I've seen some outstanding examples
    > in the antique shops and museums in Europe.
    > I understand that this is harder to read, but the whole mechanism
    > seems to me much more robust and reliable.
    > (Because there is no wormscrew-and-tooth assembly which is vulnerable
    > to wear, dust, small insects and sea salt:-)
    >
    > Alex.
    >
    > On Tue, 9 Aug 2005, Yourname Here wrote:
    >
    > > Alex,
    > >
    > > Since this was your first attempt to shoot from a small boat in rough
    > conditions, it would be intersting to know what procedures you used to get your
    > self postioned so that you were not concerned about f! alling or going overboard,
    > and what else you did to get the best sight possible.
    > >
    > > I guess your skipper was more concerned in getting to his destination.
    > Frequently, a change in course or heaving to, provides a much better platform.
    > >
    > > Joel Jacobs
    > > --
    > > Visit our website
    > > http://www.landandseacollection.com
    > >
    > >
    > >
    > >
    > > -------------- Original message from Alexandre E Eremenko
    > : --------------
    > >
    > >
    > > > For the first time I had an opportunity to use
    > > > the natural horizon.
    > > > I sailed from Dublin to Kiel, with stops in
    > > > the Scilly Islands, the Isle of White and Helgoland,
    > > > in a 42 feet sailboat, on July 23-August 6.
    > > > Unfortunately, the weather was not good fo! r Cel Nav.
    > > > For several days the Sun was not visible at all,
    > > > on other days I had to use small gaps in the clouds.
    > > > There was no oppornutity for Lunars, and I could make
    > > > only one star observation, in unfavorable conditions.
    > > >
    > > > The first week was a disappointment (from the Cel
    > > > Nav point of view); we sailed downwind with hudge waves,
    > > > and I could only make observations without a scope.
    > > > The results were very bad, but one has to take into account
    > > > that these were my FIRST observations with natural
    > > > horizon.
    > > >
    > > > One general remark on observations without a scope.
    > > > I think, one HAS to use the zero-magnification tube.
    > > > Otherwise, it is hard to make sure that your line
    > > > of sight is parallel to the pl! ane of the arc,
    > > > especially when the boat is shaking violently, and
    > > > its course is not stable.
    > > > This introduces a hudge "colimation error"
    > > > which resulted in errors in position line of several minutes,
    > > > once even 2 degrees, though I am not sure what exactly
    > > > caused such blunder.
    > > >
    > > > In the second week, I achieved a tolerable precision.
    > > > Maybe because of some practice, but more likely,
    > > > because the weather improved.
    > > >
    > > > Here is the statistics of all my observations in the second
    > > > week (August 1-6).
    > > > I used my SNO-T sextant and Casio watch. Reduction with
    > > > the Almanac and Casio calculator.
    > > > The number in parentheses is the number of observations
    > > > averaged; if there is no number > > > in parentheses, this means that only one observation
    > > > was made. The number of minutes
    > > > is the error
    > > > in altitide (using GPS position as AP).
    > > > August 1 Sun, 30 knots wind, waves 2-3 meters,
    > > > course downwind: (5) -0.2'
    > > > August 2 (Sun from a beach, Helgoland)
    > > > (5) +0.5'
    > > > (1) 0.0'
    > > > (2) +0.5'
    > > > (3) +0.5'
    > > > August 4 Sun, good weather (15-17 knots wind, waves less
    > > > than 1 meter): -0.1', -0.1',
    > > > 0.1', -0.5',
    > > > 0.3', -1.2', -0.8', -0.6', (3) -0.3'.
    > > >
    > > > The only star observation was on July 25:
    > > > Venus (4), error -1.4' and Arcturus (1) error +3'.
    > > >
    > > > If anyone is interested, I can post (or send) the full detail
    > > > of each ob! servation.
    > > >
    > > > I also tested my new Bris sextant which I bought at
    > > > Cassens-Plath factory which I visited early in July.
    > > > They said that the one I bought was the last one,
    > > > its production being discontinued.
    > > > I will write a separate message on the Bris sextant.
    > > >
    > > > Alex.
       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site